Most practical printing technologies can be roughly divided into three categories: (1) those that utilize some kind of printing member on which the image to be printed is recorded or impressed more or less permanently prior to the printing operation, (2) those that employ a photo- or electrosensitive element upon which the image to be printed is recorded and colorant is applied (or generated) prior to each printing operation, and (3) those that involve some type of plateless, direct imagewise colorant transfer from a "donor" element or reservoir to a receiving medium to create each print. Examples of the first type of printing include offset printing (i.e. lithography), letterpress printing and common rubber stamps. Examples of the second type of printing include xerography, electrophotography and electrography, and examples of the third type of printing include ink-jet, laser and thermal dye transfer printing. Lithography can involve the use of "wet" or "dry", and conventional processing or "processless", imaging techniques.
Each of these printing technologies has advantages and disadvantages for specific printing applications. Thus, each individual technology generally is limited to specific printing applications.
The art of lithographic printing is based upon the immiscibility of oil and water, wherein the oily material or ink is preferentially retained by the image area and the water or fountain solution is preferentially retained by the non-image area. When a suitably prepared surface is moistened with water and an ink is then applied, the background or non-image area retains the water and repels the ink while the image area accepts the ink and repels the water. The ink on the image area is then transferred to the surface of a material upon which the image is to be reproduced, such as paper, cloth and the like, either directly or by using a blanket roller.
Aluminum has been used for many years as a support for lithographic printing plates. In order to prepare the plate for use, it is typical to subject it to one or more treatments to improve adhesion of radiation-sensitive materials, and to enhance the water-receptive characteristics of the support. A wide variety of radiation-sensitive materials suitable for forming images for use in the lithographic printing process are known for application to the noted supports.
Such lithographic printing plates are not readily reused. Reuse requires expensive and labor-intensive removal of residual imaging materials and layers, as well as residue from support treatments. In order to clean the capillaries in the surface of the treated support of such plates, deep-acting cleansers must be used in a lengthy cleaning process.
Moreover, lithographic printing plates of the type described above are usually "wet" processed using an alkaline developing solution after imagewise exposure. The developing solution, which is used to remove the non-image areas of the imaging layer, frequently includes a substantial amount of organic solvent. The need to use and dispose of substantial quantities of alkaline developing solution has long been a health and environmental concern in the printing art. Thus, efforts have been made for many years to provide a means for printing that does not require the use of an alkaline developing solution.
Lithographic printing plates designed to be used without such solutions have been proposed in the patent and technical literature. Some are commercially available. Thus far, they have suffered from one or more disadvantages which limit their usefulness. For example, some plates have lacked a sufficient degree of discrimination between oleophilic image areas and hydrophilic non-image areas with the result that image quality on printing is poor. Other plates have had oleophilic image areas which are not sufficiently durable to permit long printing runs. Still other plates have had hydrophilic non-image areas that are easily scratched and worn, or they have been unduly complex and costly by virtue of the need to coat multiple layers on the support. Some "wet processless" printing systems require the use of donor and receiver elements, or need rubbing or complicated debris removal equipment.
The lithographic printing plates described hereinabove are printing plates which are employed in a process which employs both a printing ink and an aqueous fountain solution. Also well known in the lithographic printing art are "waterless" printing plates that do not require the use of a fountain solution. Such plates have a lithographic printing surface comprised of oleophilic (ink-accepting) image areas and oleophobic (ink-repellent) background areas. They typically comprise a support, a radiation sensitive layer that overlies the support, and an oleophobic silicone rubber outer layer, and are subjected to the steps of imagewise exposure to form the lithographic printing surface. Lasers are typically used for imaging. In such instances, the laser imaging conditions "ablate" or partially or totally remove one or more layers in the image areas of the printing plates.
While such imaging methods are useful in many instances, there is a need to dispose of the "ablated" debris from the image areas. This can be done by wiping, washing, vacuum or other mechanical means. This step, while essential in such methods, complicates the imaging and printing processes, requiring more complicated imaging equipment and/or cleaning solutions. Hence, there is a desire in the art to avoid the use of "ablation" imaging if possible for this reason.
Another problem with "ablatable" printing plates is that they cannot be reused, and they still present an environmental problem with disposal of debris and the plate itself after printing. Researchers have been considering printing materials from which the image can be "erased", and the plate thereby reused. Erasable zirconia ceramic imaging materials and methods are described, for example, in copending and recently allowed U.S. Ser. No. 08/576,178 (filed Dec. 21, 1995 by Ghosh et al) now U.S. Pat. No. 5,743,188 issued Apr. 28, 1998.
Erasable printing members composed of ferroelectric materials and useful for offset printing are described in U.S. Pat. No. 5,454,318 (Hirt et al), U.S. Pat. No. 5,555,809 (Hirt et al), CA 2,157,810 (Weiss et al) and by Hirt et al, Integrated Ferroelectrics, 10, pp. 319-326 (1995). Such printing members are used when hydrophobic and hydrophilic areas are formed in an imagewise fashion on a ferroelectric material from irradiation. This material can be polarized and depolarized in selected areas or can be brought into the three different polarization states (positive or negative polarization, or depolarization). A printing member is polarized by applying an electrical (D.C.) voltage to an electrode and using an electrically conductive layer beneath the ferroelectric material as a counter-electrode. Alternatively, the printing member can have an outer layer having strong micro-dipoles. In the U.S. patents, various overcoat materials are applied to the printing member in image areas to provide greater wearability. Various materials, such as barium titanate, lead zirconium titanates or a composite material embedded with ferroelectric crystallites, that have ferroelectric properties are well known for this purpose.
These described imaging members have the disadvantage, however, in that they require a more complicated and cumbersome imaging procedure. For example, an electromechanical step is required for electrical polarization of the imaging member.
The integration of ferroelectric ceramic layers with single-crystalline microelectronic elements has been used to make infra-red detectors and what are known as microelectronic machines (MEM's). They have also been used for microelectronic memory elements or devices (Fe-RAM) that require no applied voltage or refresh signal to maintain their polarization.
It would be desirable to have a printing technology that can be readily adapted to a wide variety of printing applications, and that has various advantages over the several technologies described above. Thus, it would be desirable to have a printing member that can be used to deliver a unique image for each printing step, or provide a number of identical prints of the same image without reexposure or reimaging before every printing step. It would also be desirable to avoid the environmental, health and operational problems accompanying the more conventional lithographic printing technologies and printing materials described above. For example, it is desirable to avoid the need for post-imaging processing solutions, for cleaning the supports before reuse, or the need to dispose of imaging debris. The printing members should also be simple to use, erasable and reusable.